The preparation of radiographs using X-rays and other such penetrating radiation is well known. Several means for detecting X-rays include use of photographic, fluorescent, ionizing and photoconductive effects thereof. Photographic methods involve exposure of a conventional silver halide X-ray film, or X-ray film coated with a fluorescent material, to the X-rays, followed by development of the film. Silver halide films presently are expensive, and their processing is lengthy and environmentally damaging. Furthermore, supplies of silver are limited and the cost thereof can be expected to increase greatly in the future.
In ionography, X-rays are absorbed in a gas layer across which a high electric field is maintained. Ions produced in the gas by the X-rays are accelerated by the electric field onto a dielectric imaging surface where an image of electrostatic charges is formed, which charge image then is rendered visible by toning. For high resolution imaging, the gas gap in which the X-rays are absorbed must be kept as narrow as possible to limit diffusion and scattering of ions as they migrate through the gas to the dielectric imaging surface. However, a narrow gas gap is very transparent to X-rays. To achieve appreciable interaction and ion formation, the use of a heavy gas having a high atomic number and maintained at a high pressure have been found necesssary. Often, xenon gas is used at pressures of approximately 10 atmospheres. The gas is expensive, and difficulties arise in the design and construction of suitable exposure cells capable of handling such expensive and high pressure gas. A radiographic system employing ionography is disclosed in U.S. Pat. No. 3,774,029 by Eric P. Muntz, et al.
In charge transfer radiography, X-rays are absorbed in a thin photoconductive layer, such as selenium, to form electron-hole pairs within the layer. A dielectric film, supported by paper or X-ray-transparent sheet, is pressed into uniform contact with the photoconductive layer. An electric field transfers charge from the surface of the photoconductor to the surface of the dielectric, which charge is rendered visible by toning. It will be noted that in a similar xeroradiographic process, a charge is deposited initially on the photoconductor surface and then discharged by the incident X-ray beam. Charge transfer radiography is disclosed in an article entitled "Charge Transfer Radiography Using Cadmium Sulfide" by I. Brodie, R. A. Gutcheck and J. B. Mooney appearing in Application of Optical Instrumentation in Medicine VIII, Vol. 233 pages 65-74, dated Apr. 20-22, 1980. Also, a charge induction electrophotographic imaging process which employs an insulating mesh between the photosensitive layer and the final record member is disclosed in U.S. Pat. No. 3,653,890 by Seimiya et al.
With charge transfer radiography and xeroradiography a photoconductive layer of sufficient thickness to absorb an appreciable fraction of X-rays is indicated. However, due to recombination processes within the photoconductor, increases in thickness of the photoconductive film beyond, approximately, 100 to 200 .mu.m do not result in more charge being transferred to the dielectric imaging surface. Also, particularly in the low-density, lower contrast areas of the image, the resolution is poor and is dominated by surface structure effects of both the dielectric imaging surface and photoconductor surface which results in a generally mottled appearance.